A secondary liquid-electrolyte battery generally contains at least one electrochemical cell that includes a negative electrode, a positive electrode, and a separator situated between the electrodes. The negative and positive electrodes are constructed from materials that can participate in both oxidation and reduction reactions. Such electrode materials allow an electric current to be reversibly passed between the electrodes in an external circuit, while an ionic species migrates within the cell, between the electrodes through the separator within a liquid electrolyte to electrochemically balance the external electron current flow. These concurrent electrical current and ionic current flows occur spontaneously during cell discharge. During cell discharge, oxidation occurs spontaneously at the negative electrode and reduction occurs at the positive electrode. Reverse reactions are compelled during the cell charge phase in which oxidation occurs at the positive electrode and reduction occurs at the negative electrode. The electric current generated during cell discharge may be used to power, at least in part, an electrical load, while an applied voltage from an external power source may be used to charge, or re-power, the cell once its current capacity has fallen to an undesirable level.
Lithium-ion electrochemical cells, for example, are used in many secondary, liquid-electrolyte batteries. In one example of a lithium-ion cell, the electrolyte comprises a suitable lithium compound, such as LiPF6, which is dissolved in a non-aqueous organic liquid. The anode may comprise a thin layer of graphite suitably bonded to a thin metal current collector strip. Lithium is intercalated into the graphite layers during charging of the cell. And the cathode comprises a thin layer of a suitable lithium compound, also bonded to a thin metal current collector strip, for receiving lithium ions transported from the anode through the electrolyte to the cathode during discharge of the cell. A thin porous polymer separator membrane is placed between the anode and cathode layers.
The separator facilitates operation of the electrochemical battery cell by providing a porous and electrically-insulative mechanical support barrier between the two electrodes. The separator, in general, has a porosity sufficient to contain the liquid electrolyte—which can transport the ionic species between the electrodes—yet is thermally, chemically, and mechanically stable enough to separate the closely spaced, opposing negative and positive electrodes over the course of many discharge/charge cell cycles so that a short-circuit is prevented. A wide variety of materials, either alone or in combination with one another, have been either utilized or investigated for construction of the separator with the goal of imparting long term operational reliability to the separator within different working environments. The most commonly used separators today are made from a single flat polyolefin sheet membrane or a laminate of several flat polyolefin sheet membranes. The particular polyolefins usually employed are those derived from simple low-carbon-number olefins, such as polypropylene and polyethylene.
The electrochemical battery cell, in order to interact with the electrical load and the external power source, is configured for connection to an external circuit that provides an electric current path between the negative and positive electrodes external to the electrochemical cell. Each of the negative and positive electrodes, for instance, is typically associated with a metallic current collector that helps distribute the electric current passing through the external circuit to and from all electrochemically active regions of the electrodes. A connection feature such as a connector tab may be included on each of the metallic current collectors. The connection feature may protrude away from the electrochemical battery cell to operatively establish an electrical connection with the external circuit. This is usually accomplished by connecting the protruding connection features associated with the negative and positive electrodes to negative and positive terminals, respectively, in either a serial or parallel relationship with the connection features associated with other electrochemical battery cells. Negative and positive terminals may not be needed, however, if the secondary liquid-electrolyte battery includes only one electrochemical battery cell.
In automotive vehicle applications, for example, many lithium-ion electrochemical cells may be interconnected in series and parallel electrical circuit connections to form a secondary battery that is capable of delivering substantial electrical power at a relatively high voltage to a traction motor for driving the vehicle. The battery is subject to ambient heating and cooling in the vehicle environment. In this application, the thin electrode and separator elements of each cell also experience heating due to significant power load demand and are subjected to many repeated discharge and re-charge cycles. But, more critically, the battery may experience significant heating under abusive conditions. Thin polyolefin separators, for example, may be exposed to elevated temperatures which soften them and reduce their effectiveness in maintaining suitable separation of closely spaced anode and cathode layers. The shrinkage experienced by a polyolefin separator at an elevated temperature can also increase the risk of battery electrical shorting. There is a need to consider other materials which can be formed into strong, temperature resistant, and electrochemically effective separator membranes or thin layers.